1. Field of the Invention
This invention relates to a method and an apparatus for predictive coding usable in a band reduction of a color video signal.
2. Description of the Prior Art
Band reductions are sometimes used in transmission of video signals. In known band reductions of a color video signal having a chrominance subcarrier, after the color video signal is sampled at a frequency equal to or higher than twice the chrominance subcarrier frequency and is converted into a corresponding digital signal, the digital color video signal is subjected to a predictive coding process.
In the case of a color video signal with a high picture quality, since the band of the color video signal is relatively wide, the sampling frequency is required to be high and thus difference calculation for the predictive coding process tends to be difficult. Japanese published unexamined patent application No. 58-147288 discloses a method of predictive coding which is intended to resolve this problem.
In the predictive coding method of Japanese patent application No. 58-147288, after a color video signal is sampled at a frequency equal to an integer "n" (a natural number greater than one) times the chrominance subcarrier frequency and is converted into a corresponding digital signal, every n-th of the sampled data is successively supplied to a predictive encoder for prediction. In this case, since the time interval between the data from which a predictive value is derived and the data which is to be compared with the predictive value is in agreement with the spatial distance between the corresponding picture elements in a transmitted picture, accurate predictions tend to be difficult for abrupt edge portions in the picture. In such abrupt edge portions, a predictive value and an actual value are greatly different from each other and thus the difference therebetween is large so that a high bit rate tends to be required for accurate transmission. When a low bit rate is used as in general transmissions of video signals, the picture quality of such abrupt edge portions in a reproduced picture is sometimes unacceptable.